Compound A has an excellent antibacterial activity (see, for example, Patent Document 1) and has been placed on the market as a synthetic antibacterial agent. It is known that there are 3 types of crystal forms (type I, type II and type III) of Compound A, and they are designated as type I, type II and type III in descending order of melting temperature in the measurement by differential scanning calorimetry (hereinafter referred to as DSC) (see, for example, Non-patent Document 1). In addition, by considering the solubility, absorbability, therapeutic effect and the like of the respective crystal forms, the type III crystal thereof has been placed on the market (see, for example, Non-patent Document 1).
It is known that the type I, type II and type III crystals of Compound A are obtained by crystallization from acetonitrile, however, the conditions of crystallization thereof have not been known, and existence of Compound B have not been known either (see, for example, Non-patent Document 1).
Patent Document 1: JP-A-1-294680
Non-Patent Document 1: Kazuro Kakemi and others 7, “Chemical Structure, physicochemical properties and stability of Prulifloxacin”, IYAKUHIN KENKYU, vol. 28(1), pp 1-11 (1997)